I had a conversation recently with a good friend in which we were discussing evidence for a Trinitarian reading of the Bible. At some point, I made the statement that the Trinity was so essential to my concept of reality that my world would come apart without it; the doctrine of the Trinity makes the world make sense. My friend then said that he had heard others say similar things and asked for an explanation. Why would I take the world to be unintelligible apart from the presence of the specifically triune God?
My initial response to this question is bound up with the notion of personhood. What does it mean to be a person? To answer this question, we must realize that a significant historical development in the concept of personhood was nothing less than the doctrine of the Trinity. The early church fathers needed a way to speak of both the unity and distinction that describes the God revealed in Jesus Christ and Christian scripture. With regard to the divine unity, they turned to the language of essence or substance. With regard to the distinction between Father, Son, and Holy Spirit, they turned to the language of person. Before this development, the Latin persona referred to the masks that actors wore during a stage performance but hardly carried any sense of the modern concepts associated with personhood or personal identity. But when the language of person was used to describe the relationships between the members of the Trinity, that language took on a whole new significance. We began to understand that to be a person meant to be a person in a relationship of other-oriented love. And as Dennis Kinlaw suggests, "With this new understanding of the nature of God comes a new concept of love as well - a love determined by the nature of the subject who loves rather than by the nature of the object loved" (Let's Start with Jesus, 29) The notion of loving another not for what you could get but for who you are became attached to the language of person. And the world changed.
So, why is the Trinity essential to my view of the world? Why would my world collapse if God were not triune? Because my understanding of what it means to be a person is bound together with my understanding of God as a single divine essence of three persons in relationship. The members of the Trinity exist eternally in a personal relationship of other-oriented love. That reality gives meaning to my understanding of myself and others as a persons. That reality explains the way I conceive of my relationships. Indeed, my conception of my marriage as two equal persons in a covenant of other-oriented self-giving love is incomprehensible if the Father, Son, and Spirit are not eternally relating to one another as persons in a covenant of other-oriented self-giving love. If personhood is about being in a relationship where the other is loved for who they are and not for what you can get from them, then the concept of personhood is unintelligible in a world where the Father does not eternally love the Son. I do not think the concept of personhood would have come about if we lived in a world of polytheism or unititarian monotheism. We are only able to understand ourselves as persons because we are made in the image of the God who is eternally three persons who share one essence.
This argument could be summed up by adapting the quote from C. S. Lewis in which he said, "I believe in Christianity as I believe that the sun has risen, not only because I see it, but because by it I see everything else." I believe in the persons of the Trinity, not only because I see them through revelation, but because by that revelation I can make sense of myself and others as persons in authentic relationships.
For more along these lines, see Dennis Kinlaw's Let's Start with Jesus: A New Way of Doing Theology (Zondervan 2005).